DFT computational and experimental study of indole continuum solvation

A combined DFT and experimental study of the structure and the harmonic vib rational frequencies of free and solvated indole was performed. The DFT stu dy included full geometry optimizations, as well as numerical harmonic vibr ational analyses at BLYP/6-31G** level of theory. The solvent influence was included via the SCRF methodology (within the Onsager model). The FTIR spe ctra of indole solutions in CCl4 were recorded and compared with the theore tical results. The BLYP formalism was shown to be very suitable for vibrati onal assignments without scaling in the 1600-1000 cm(-1) frequency range. T he computed higher frequency modes (3500-3000 cm(-1)) overestimate the expe rimental values, and scaling with a constant factor of about 0.98 is requir ed. The continuum solvation has only a subtle influence on both the structu re and harmonic force field. However, especially for the frequency range in which no scaling is needed, the gradient-corrected form of the DFT methodo logy is much more useful for vibrational assignments than the standard HF p rocedures that include scaling. On the basis of these more accurate DFT cal culations, several reassignments were suggested in the vibrational spectra of the vapour specimen (compared to the HF/3-21G ones), which improved the convergence of the subsequent force fields fitting. (C) 1999 Elsevier Scien ce B.V. All rights reserved.